Replicas for use in the production of refractory molds



Patented Feb. 17, 1953 UNITED STATES TENT OFFICE Ilsabe E. Valyi, NewYork, N. Y., assignor to Emerik Imre Valyi, New York, N. Y.

No Drawing. Application June 17, 1950,

Serial No. 168,860

4 Claims.

This invention relates to the precision casting of metals. Moreparticularly, it relates to new and improved replicas (patterns) for theproduction of refractory molds in precision casting of metals, and to anew and improved composition from which such replicas are produced.

This application is a continuation-in-part of my co-pending applicationSerial No. 688,335, filed August 3, 1946, for Replicas for Use in theProduction of Refractory Molds, and Compositions Therefor, nowabandoned.

Metal castings have been made by forcing a molten metal or alloy into amold made from a refractory composition. The mold was usually made bycasting a so-called investment of the refractory about a replica of wax.After the refractory had been set, the wax replica was melted andremoved. The set refractory was finally subjected to a high temperatureor firing whereby it was brought into final shape and condition to beused as the mold for metal casting.

In principle, the procedure of using the wax replicas above describedappears to be simple and attractive. However, in practice it has manyshortcomings and disadvantages. Though wax can be easily cast or moldedto form the replica, nevertheless such replica does not possesssatisfactory dimensional stability. The wax patterns are sensitive totemperature, organic solvents and mechanical actions. At temperatures of50 F. and below, which may well be met in manufacturing practice,molding waxes tend to become extremely brittle, with the consequencethat much breakage of replicas formed thereof takes place duringhandling. Additionally, at higher temperatures, the wax replicas becomesoft and easily undergo plastic deformation under the influence of smallstresses, resulting in loss of dimensional accuracy in the final mold.Furthermore, replicas of wax undergo undesirable ageing effects uponstorage. In many cases, even manual handling of such replicas deformedthem so as to render them unsuitable for use.

Attempts have been made to overcome the defects of wax replicas byforming the replicas from high molecular weight commercial resins.However, replicas of such materials were unsatisfactory in that theytended to warp and deform during the casting, setting, and drying of theinvestment mold. Additionally, when such high 2 molecular weight resinswere injection-molded, it was necessary to use steel molds to withstandthe high molding temperatures and pressures which were required.

An object of this invention is to provide a new and improved replica forthe production of a refractory mold to be used in precision casting ofmetals and alloys.

Another object of this invention is to provide a replica which undergoessubstantially no deformation during the production of a refractory moldto be used in precision casting of metals and alloys.

An additional object of this invention is to provide a replica for theproduction of a refractory mold to be used in precision casting ofmetals and alloys and which is characterized by high chemical andmechanical stability.

A further object of this invention is to provide a replica which doesnot crack the mold due to thermal expansion.

Another object is to provide a replica having improved thermal expansioncharacteristics.

A still further object of this invention is to provide a replica havingthe above characteristics which can be molded at lower pressures andtemperatures than the usual high molecular weight resins.

Other and additional objects of the invention will become apparenthereinafter.

The above objects are accomplished, in general, by producing the replicafrom resins having a sufiiciently low molecular weight to impart theabove characteristics.

Although replicas consisting of the low molecular weight resins aresatisfactory, fillers or plasticizers, or mixtures thereof, can beincorporated into the low molecular weight resin if desired, and suchcompositions employed for the production of replicas.

In practicing this invention, to produce a desired metal casting thereis first prepared a metal die from a positive. This die, which may beformed of a soft metal or alloy, such as tin babbit, lead babbit, orpreferably a zinc base alloy or similar soft metal, is produced bycasting and subsequent pressing and is utilized to produce any number ofthe low molecular weight resin replicas. Each replica or plurality ofreplicas, as

may be necessary, may be appropriately mounted ed es.

by the use of a sprue forming extension and a flask is placed around theset-up. The mold composition is introduced into the flask and the flaskis left at room temperature or subjected to a slightly elevatedtemperature until the mold composition has set. Thereafter, the flash isplaced in an oven for firing the mold. During the firing cycle, thereplica undergoes combustion, leaving a cavity in the mold correspondingprecisely to the shape of the replica. The fired mold is then removedfrom the oven and used in the desired metal molding :or castingprocedure.

The details and manner of practicing the invention will become apparentby reference to the following specific examples, it being understoodthat the examples are merely-illustrativeembodiments of the inventionand that thezscopeof the invention is not limited thereto.

Example I A die made from a zinc base alloy was pre pared in the usualway by casting such :allqy around a steel positive and followed up byrepressing to insure perfect surface and sharp The die consisted of twoparts which fittedtogether and provided a'cavity of the.desired-shapebetween them. This cavity wascom- --plete1y enclosedexcept for'a channel to the outside, said channel consisting of a groovein each zinc base-alloy face.

--Polystyrene having an intrinsic viscosity tin toluene of "0.142,corresponding to a viscosity average molecular weight "of 9500 "washeated I in a cl'l'amber"to200 'Cxand forced into "the die l through theabove channel under a drivingpresi "sure of approximately 250 pounds persquare inch.

Thedie was split'open, and the injection molded, polystyrene replicaremoved.

The replica was easily removed from the die with no tendency towardadherence to the .die

' surface. "-It possessed sufficient toughness for use as a replicainthe precision casting process.

No deformation or warpage were observed when the replica was exposed totemperatures up to 45 'C., a temperature at which a'replica made fromthe usual commercial molding grades (high mdlecularwei'ght)polystyrenewould deform extensively. No sinks or depressions were formed 'in the'fiat surfaces even on parts of heavy crosssection.

On firing of the replica, vapors were evolved on decomposition. However,combustion proceeded smoothly, with no harmful changes .in

shape' and withoutproducing any cracks in the ing to a viscosity averagemolecular weight of 25,000 was forced into adie, prepared-as set forthin EXampleI, at a temperature of approximately :200 C. and undera-driving pressure of approxi- Y matelyB'OO'pounds per square inch. Thediewas opened and, after the molded replica had been :stripped, severalof them were assembled in a flask and invested. The mold thus formed wassubjected to the usual drying and firing cycle,

during which the polymethyl methacrylate replica burned out.Stainless'steel castings made on :a centrifugal casting machine from therefractory'mold'had very good surfaces and their dimensions showed thatthe plastic replica had suffered no deformation and no dimensionalchanges other than thermal-expansion.

Example III Using a similar die and procedure as in Example I,polystyrene, having an intrinsic viscosity in toluene of 0.20,corresponding to a viscosity average molecular weight of 13,000 wasmolded under a pressure of approximately 300 pounds per square inch andat a temperature of approxi mately 190 0.

Example IV Example V Same as Example I, except that polystyrene, .havingan intrinsic viscosity in toluene of 0.281, corresponding to a viscosityaverage molecular weight of 19,000, was molded at a temperature of about200 C. under a driving pressure of approximately 350 pounds per squareinch.

.Erample VI Same as Example 1, except that a polyethylene sample ofmolecular weight 10,000, into which had'been incorporated by .weight'ofparaffin wax, was molded at a temperature of approximately '90 C. under'a driving pressure of approximately 200 pounds per square inch.

Additional examples 1 Intrinsicvismsity in'toluene.

2 Visc sity averge molecular weight. M ldiag temper ture.

' Molding pressure in pounds per-square inch.

Replicas according to'all of the above examples were found to have thecharacteristics enumerated under Example I.

As seen from the examples, polystyrene having an intrinsic viscosity intoluene of the range 0.14 to 0.75 may be used. However, the range of0.40 to 0.65 is preferred forreasons of molding economy.

This entire intrinsic viscosityrange is well "be low that of the usualcommercial injection molding grades of polystyrene which are of theorder of 1.3 to 2.0.

'The invention 'is not restricted to the specific low molecular weightresins disclosed in the examples. In general the following low molecularweight resins in molecular weight ranges sufficiently low to impart thedesired characteristics may be used:

1. Copolymers of styrene with other chlorinefree monomers;

'2. Acrylate-type polymers, such as polyethyl methacrylate and polyethyl'acrylate;

3. Chlorine-free vinyl resins, such as polyvinyl acetate; and

'4. Cellulose derivatives, 'such as cellulose acetate, cellulose acetatebutyrateyand cellulose acetate propionate.

While replicas'acc'ording totheabove examples are satisfactory in mostinstances, the properties may be further improved by the addition ofsuitable plasticizers which have been found to reduce the moldingtemperature or pressure.

Such plasticizers also decrease the second order transition temperatureof the resin composition so that the rate of expansion of thecomposition remains essentially linear in the critical mold settingtemperature range. This prevents sudden expansion of the replica whichis the major cause of mold cracking. The plasticizers must leave noresidue in combustion and must be inert to the investment composition.

For polystyrene the following plasticizers have been found useful:

The plasticizers may be used with the resin compositions of Examples I,IlI-V and VII-X in proportions from 0 to 40%, preferably .in the rangeof 10% to The molding temperature and pressure is reduced as a functionof the plasticizer content. The preferred range has been found to effectthe desired lowering of the second order transition temperature. I

In any of the compositions the molding temperature varies inversely withthe molding pressure and any suitable combination may be selected.

As indicated above the polystyrenes have been found particularlysuitable.

Optionally fillers may be incorporated in the low molecular weightplasticized or unplasticized resin composition from which the replica ismade. In general, when a filler, such as carbon black, wood flour, etc.is used, it may constitute from 10% to 40%, preferably from 10% to 20%,by weight of the composition.

The low molecular weight resins are prepared by any of the followingmethods: For example, the low molecular weight polystyrenes andpolymethacrylates can be obtained by polymerizing the appropriatemonomer in dilute solution with high catalyst concentration at hightemperatures. Preferably, the low molecular weight polystyrene andpolymethacrylates are prepared by the addition of a chain-transferagent, such as carbon tetrachloride, to the polymerizing system. Thefollowing are illustrative examples of such method:

(A) A mixture of 50 parts by weight of styrene monomer, 50 parts byweight of carbon tetrachloride, and 1 part of benzoyl peroxide is heatedat a temperature of 80 C. for 6 to 8 hours. The resulting polymer isisolated from the reaction mixture and dried.

(B) A mixture containing 500 parts of water, 100 parts of styrenemonomer, parts of carbon tetrachloride, 1 part of benzoyl peroxide, anda small amount of a suitable suspending agent is heated with agitationfor 10 to 12 hours under reflux conditions. The resulting polymer issubjected to steam distillation and drying.

(C) A mixture containing 100 parts of styrene monomer, 1 part of dodecylmercaptan and 0.4 gram of benzoyl peroxide was heated for 12 hours at C.The resulting polymer is removed from the container and ground intosmall particles.

The molecular weights mentioned herein were derived from intrinsicviscosity measurements.

The definition of the intrinsic or limiting viscosity is well known tothose familiar with the art to be hu) 1] r that is, the limiting valueof the specific viscosity divided by the solution viscosity where thelatter approaches zero. I

[1 was determined by measuring the specific viscosity 1; so1ution1solvent 1 solution of several highly dilute polymer solutions in a givensolvent, dividing the specific viscosities by the concentrations atwhich they were measured, and plotting the values so obtained againstconcentration. In this plot of 71sp/C versus 0 a straight line isobtained whose intercept with the viscosity axis represents theintrinsic viscosity.

The intrinsic viscosity was translated into a molecular weight value bymeans of the equation wherein the constant K was assumed to have thevalue 1.5 10- Other equations and other constants can be used to relatesolution viscosity and molecular Weight. The molecular weight will varyaccording to the equation and constantsused. The intrinsic viscosity,however, is a definite characterization of a given polymer. r

The replicas of this invention can be used with any of the knowninvestment compositions. Investment compositions of the type disclosedin application Serial No. 644,420, filed January 30, 1946, forInvestment Compositions and Method of Producing Refractory MoldsTherefrom, now Patent No. 2,521,614 can also be used.

The replicas can be produced by any molding or casting process, such ascompression-molding, injection-molding, etc."

The use of a low molecular weight resin as distinguished from the highmolecular weight resins commonly used 'commerciallyfor injectionmolding, results in the following advantages:

1. Due to the lower softenin temperatures of thelow molecular weightresin the patterns become softened and drain out of the mold before theyhave reached a high temperature. This results in a lower total expansionof the pattern within the mold as the temperature of the investment israised.

2. The lower total expansion permits large pattern sizes to be usedwithout cracking the investment or otherwise injuring the surface of themold. i Q

3. The plastic drains out of the mold at such a low temperature thatpractically all of the combustion can be made to occur outside the mold.cavity. This results in a cleaner mold cavity and an improved surface ofcastings.

4. The low molecular weight resins described above may be molded at muchlower temperatures and pressures than the high molecular weight resins.For this reason the production of the pattern is simplified in that theclamping pressures of the machinery, used to mold such low molecularweight resins, are materially reduced, resulting in great economy ofoperation.

"As another result of the lower molding tempera! ture and pressure, wearon and deformation of die cavities is reduced and dies made of metalsother than steel may be used. The cost of such dies mayoften be but afraction of the cost of a steel die.

5. The low molecular Weight resin patterns have less internal strainsand, hence, there-is a reduced tendency to distort when they are raisedto the softening temperature. This results in a more accurate castingthan in the case of high molecular weight patterns which have highinternal strains and tend to warp during the setting and drying of theinvestment. 7

6. In commercial operations the combination of the above qualitiesresults in a very low reject rate both for the patterns andfor the castings.

7.. Due 'to thelower-molding pressure required for the. low molecularweight resins, a die clamping force of only about one-tenth of the forcenecessary for molding high molecular weight resins is required. Thisreduces the cost of the injection molding machinery to one-third of thatrequired for conventional high molecular weight plastics.

It is obvious that various changes and modifications can be made in theabove description without departing from the nature or spirit theinvention. I v

What is claimed is:

' 1. The method of producing refractory mold for industrial precisioncasting, which comprises embedding in an investment composition aplastic replica consisting essentially of polystyrene having anintrinsic viscosity in toluene of 0.142 to 0.75, corresponding to acalculated viscosity average molecular weight of 9,500 to 50,000, saidreplica having mold cavity forming parts with exterior surfacesconforming to the exterior surface of the article to be cast, having theproperty of being rigid at room temperature, softening at highertemperature with thermal expansion in an amount which is insufficient toinjure the mold, and being completely combustible at mold firingtemperature, subjecting the assembly to conditions of temperature andtime to cause solidification of the investment,

removal of the replica by softening and combustion to leave a moldcavity, and conversion of the solidified investment to a refractorymold. 2. The method of producing refractory molds for industrialprecision casting, which comprises embedding inan investment compositiona plastic replica consisting essentially of polystyrene having anintrinsic viscosity in toluene of 0.40to 0.65 corresponding to acalculated viscosity average molecular weight of' 26,500 to 43,500, saidreplica having mold cavity forming parts with exterior surfacesconforming to the 8 investment, removal of the replica by softening andcombustion to leave a mold cavity, and conversion of the solidifiedinvestment to a refractory mold.

'3. The method of producing refractory molds for industrial precisioncasting, which comprises embedding in an investment composition aplastic replica consisting essentially of polystyrene having anintrinsic visocity in toluene of 0.142 to 0.75, corresponding to acalculated viscosity average molecular weight of 9,500 to 50,000, and aplasticizer in an amount not over 40%, said replica having mold cavityforming parts with exterior surfaces conformin to the exterior surfaceof the article to be cast, having the property of being rigid at roomtemperature, softening at higher temperature with thermal expansion inan amount which is insufficient to injure the mold, and being completelycombustible at mold firing temperature, subjecting the assembly toconditions of temperature and time to cause solidification of theinvestment, removal of the replica by-softening and combustion to leavea mold cavity, and conversion of the solidified investment to arefractory mold.

4. The method of producing refractory molds for industrial precisioncasting, which comprises embedding in an investment composition aplastic replica consisting essentially of polystyrene having anintrinsic viscosity in toluene of 0.40 to 0.65, corresponding to acalculated viscosity average molecular weight of 26,500 to 43,500, and aplasticizer in an amount of from 10% to 20%, said replica having moldcavity forming parts with exterior surfaces conforming to the exteriorsurface of the article to be cast, having the property of being rigid atroom temperature, softening at higher temperature with thermal expansionin an amount which is insufficient to injure the mold, and beingcompletely combustible at mold firing temperature, subjecting theassembly to conditions of temperature and time to cause solidificationof the investment, removal of the replica by softening and combustion toleave a mold cavity, and conversion of the solidified investment to arefractory mold.

ILSABE E. VALYI.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name. Date 2,211,689 vDittmar Aug. 13, 19402,362,507 Steinbock et a1. Nov. 14, 1944 OTHER REFERENCES High Polymers,by Mark et al., vol. III, pp.

1. THE METHOD OF PRODUCING REFRACTORY MOLDS FOR INDUSTRIAL PRECISIONCASTING, WHICH COMPRISES EMBEDDING IN AN INVESTMENT COMPOSITION APLASTIC REPLICA CONSISTING ESSENTIALLY OF POLYSTYRENE HAVING ANINTRINSIC VISCOSITY IN TOLUENE OF 0.142 TO 0.75, CORRESPONDING TOCALCULATED VISCOSITY AVERAGE MOLECULAR WEIGHT OF 9,500 TO 50,000, SAIDREPLICA HAVING MOLD CAVITY FORMING PARTS WITH EXTERIOR SURFACESCONFORMING TO THE EXTERIOR SURFACE OF THE ARTICLE TO BE CAST, HAVING THEPROPERTY OF BEING RIGID AT ROOM TEMPERATURE, SOFTENING AT HIGHERTEMPERATURE WITH THERMAL EXPANDSION IN AN AMOUNT WHICH IS INSUFFICIENTTO INJURE THE MOLD, AND BEING COMPLETELY COMBUSTIBLE AT MOLD FIRINGTEMPERATURE, SUBJECTING THE ASSEMBLY TO CONDITIONS OF TEMPERATURE ANDTIME TO CAUSE SOLIDIFICATION OF THE INVESTMENT, REMOVAL OF THE REPLICABY SOFTENING AND COMBUSTION TO LEAVE A MOLD CAVITY, AND CONVERSION OFTHE SOLIDFIED INVESTMENT TO A REFRACTORY MODE.